As in Frantz U.S. Pat. No. 3,603,154, issued Sept. 7, 1971 and Taylor U.S. Pat. No. 4,016,760, issued Apr. 12, 1977, differential flow meters, such as the Bourdon tube type of the Taylor patent, are widely used, with or without calibration, for indicating variations in the flow rate of compressed air or other gas through a line by measuring variations in the pressure differential or drop across a restricted orifice in the line. Since the flow rate varies as the square root of the pressure differential, for the flow rate to be directly readable on the flowmeter's scale, either the drive mechanism for an indicator pointer or the scale itself must be adapted to extract the square root of the pressure differentials. However, a uniform linear scale, such as illustrated in the indicator of the Taylor patent, giving direct readings of pressure differentials, usually will suffice as indicating variations in the flow rate.
In railway train consists and particularly in the now usual long freight train consists of a hundred or more cars having air brake lines connected in series to the compressed air system of the one or more locomotives, some air leakage from the consist's brake line is almost inevitable. The problem this poses is that if the consist's brake line leakage exceeds a predetermined maximum, its air brakes will not operate properly and under A.A.R. (Association of American Railroads) rules, the train cannot be moved before the air leakage has been reduced to an acceptable level. It is with a method and apparatus for indicating in a locomotive whether leakage of compressed air from the air brake line is within acceptable limits that the present invention is primarily concerned.